1. Field of the Invention
The present invention relates to a robot controller for controlling an industrial robot, and more particularly to a robot controller having a function of monitoring abnormality of a robot operation. The present invention is applicable to a robot for performing an operation by supplying energy or operational substance to an operational tool mounted on a robot, such as arc welding, sealing, laser beam machining, painting.
2. Description of Related Art
In performing an operation such as arc welding, sealing, laser beam machining, painting by using a robot, an operational tool such as a welding torch, a sealing gun, a laser machining head and a painting gun is mounted on a distal end of a wrist or an arm of a robot. An operation program is prepared and taught to a robot controller such that the operational tool moves along a desired operation path. In the teaching, I/O output commands to start and stop an action of the operational tool are additionally taught to the robot controller at an operation start position and an operation end position, respectively.
When executing the operation program, the operational tool is moved to the taught operation start position and an arc welder or a sealing device, etc. is actuated by an I/O command. Simultaneously, the robot starts an operational motion so that the operational tool moves along the taught operational path to perform the operation such as arc welding, sealing, etc. When the operational tool reaches the end point of the path, the I/O command is outputted to stop the action of the ark welder or the sealing device.
In the above operation, if the execution of the operation program is suspended by a temporary stop signal or an alarm, the processor for controlling the operation detects the temporary stoppage of the program and automatically outputs an I/O command to stop the action of the arc welder or the sealing operation, i.e., extinguish a welding torch or stop supply of sealant.
As described, in the ordinary situation, the arc welding or sealing operation which requires motion of the tool with respect to the workpiece is not performed with the toll unmoved. However, for example, there is a case where the operation program is prepared such that the operation of the arc welder is started at a start point of the welding operation by the I/O command and the robot is starts its motion after confirming an generation of arc by a conformation signal form the arc welder. In this case, if the I/O input signal conforming the generation of arc is not transmitted to the robot controller because of failure of cable etc., the operational tool remains unmoved since the arc generation signal is not received.
If the above event should occur, it raises a serious problem since the arc is generated at the same position on the workpiece with the torch stopped. The workpiece or a jig may be molten by large heart to make a large damage to the system.
A similar case may occur on a robot for use in an operation where operational substance or energy is applied to the workpiece while the operational tool is moving. For example, if paint is continued to be supplied to a painting gun in a stationary state of a painting robot, a workpiece, peripheral equipments and environment of the system are besmeared. If the paint contains highly volatile material there arises a possibility of fire.
Such event is also caused by incompleteness of a software program for controlling an operation of a robot as well as the above-mentioned physical failure such as breakage of a signal cable. For example, it may occur that the robot does not start its operation although a motion command is issued to the robot for the operation program. In this case also, generation of arc or supply of paint is continued while the robot remains stationary to cause the same problem.
An object of the present invention is to provide a robot controller capable of preventing possibility of accident caused by an abnormal stoppage of a robot by physical failure such as breakage of cables or incompleteness of an operational software for controlling a robot in execution of the operation in which operational substance or energy is applied to an object while moving the operational tool with respect to the object.
A robot controller of the present invention controls and monitors an operation performed by an operational tool mounted on a robot with the operational tool moving or stopping relatively to an object of operation. The robot controller of the present invention comprises: a motion controlling section to control the robot to move the operational tool relatively to the object; a supply device to supply energy to be applied to the object, or supply energy to operational substance to be fused and applied to the object, or supply operational substance to be applied to the object through the operational tool; a supply controlling section to issue a supply command to the supply device for actuating the supply device; a robot stoppage monitoring section to monitor whether or not the relative motion of the operational tool with respect to the object is stopped; a supply command monitoring section to monitor whether or not the supply command is issued to the supply means; a timer to measure a time period during which the relative motion is stopped and also the supply command has been issued to the supply device; and an abnormality determining section to determine an abnormality of the operation when the time period measured by said timer exceeds a predetermined time period.
It is preferable that the predetermined time period for determination of abnormality is variably set as occasion arises. For some kinds of operations, it is preferable that the predetermined time period is set differently for a start of the operation and a midst of the operation. The operational tool may be a welding torch for welding operation, a laser beam machining head for laser beam machining, a sealing gun for sealing operation or a painting gun for painting operation.
In a preferred embodiment, the system software for controlling of an operation of arc welding, sealing, laser machining etc., demands a processor of the controller to detect a start of the operation and monitor command pulses for motors for driving respective robot axes after elapse of a predetermined time period at every processing period.
If all command pulses are zero at a start of the monitoring, an I/O command is immediately outputted to stop the operation and an alarm is issued. After the monitoring is started, if it is detected that all command pulses turn to zero and remain unchanged for a predetermined time period, an I/O command is immediately outputted to stop the operation and an alarm is issued.
The reason for waiting for the predetermined time in determining an abnormality is that it is not unusual for a robot to stop its motion for a short time in a normal operation. For example, in an arc welding operation, a weaving action of moving a welding torch transversely to a welding path is often performed. In this weaving action, welding is performed with the torch stopped for a short time at transverse end points for securing sufficient pads.
In this case, if the arc is stopped immediately after all command pulses turn to zero, i.e., without a waiting time period, the arc welding operation with temporally stoppage is terminated in the midst of the operation.
Also, in the case of spot arc welding, the welding action is performed with the tool stopped for 0.5 to 1 sec. In this case also, if the arc is stopped immediately after all command pulses turn to zero, the spot arc welding is terminated in the midst of the operation.
For the above reasons, it is necessary to wait for a predetermined time period for making a determination of an abnormality. The waiting time period may be differently set for a start of the operation and a midst of the operation. For example, for the arc welding operation, it is preferable to set different waiting time periods for a start and a midst of an operation for the determination of an abnormality. Once the arc is generated, it may and should be determined that an abnormality has occurred after elapse of a short waiting time period because stoppage of the tool in a normal operation (temporary stoppage in the weaving arc welding or dwelling in the spot arc welding) is quite short.
On the other hand, at a start of generation of arc, it takes a considerable time for an arc discharge to generate by electric potential difference between a welding torch and a workpiece from receipt of a command for generating arc to an arc welder. Thus, it is necessary to set a longer waiting time period for a start of the operation for determination of an abnormality of the operation. For the above reasons, it is appropriate that the determination of abnormality is made on the basis of different criterions for a start and a midst of the arc welding operation. The different waiting time periods may be set in advance and selectively used for the start and midst of the operation.